The present invention relates to a control circuit for a DC motor, and more particularly relates to such a control circuit for a DC motor, particularly adapted to be fitted to a hand tool such as a power screwdriver or a power drill or the like.
The present inventor wishes hereby to attract the attention of the examining authorities to copending Pat. Application Ser. Nos. 936,081 and 944,220 (now U.S. Pat. Nos. 4,764,745 and 4,719,395), which may be considered to be material to the examination of the present patent application.
In the prior art, there have been proposed various types of control circuit for an AC motor, as opposed to a DC motor. Typically, an SCR circuit has been used for obtaining a speed variation control signal; but such a circuit is not usable for a DC motor. And DC motor control circuits have been generally complicated and/or bulky.
Further, in a DC motor control circuit which has a speed variation control circuit for controlling the rotational speed of the DC motor, the control power source for the speed variation control circuit is typically supplied from the primary end of a reverse switch for the motor. However, in such a conventional circuit, due to the reversion of the polarity of the motor when reversing the rotational direction of the motor, the circuit structure tends to be rather complicated, which is attendant with problems due to lack of lightness and compactness, and also is rather expensive.